Technical Field
[0001] The present invention relates to an optical fiber suitable as an optical transmission
line or dispersion compensator.
Background Art
[0002] The following optical fibers have conventionally been known, for example. A microstructured
optical fiber disclosed in JP 10-95628A has a core region, which is usually solid,
surrounded by a cladding region that comprises a multiplicity of spaced apart cladding
features that are elongate in the axial direction and disposed in a first cladding
material. The core region has an effective diameter d
0 and an effective refractive index N
0. Each cladding feature has a refractive index that differs from that of the first
cladding material, and the cladding region has an effective refractive index that
is less than N
0. Further, it is disclosed that a large dispersion is obtained since the cladding
region comprises an inner cladding region having an effective refractive index N
cl and an outer cladding region having an effective refractive index N
co (where N
cl < N
co).
[0003] OFC'96 Technical Digest, ThA3 discloses an optical fiber having a W-shaped refractive
index profile, and discloses that a low chromatic dispersion (with a large negative
value) can be realized in this optical fiber.
[0004] Electronics Letters, vol. 18, pp. 824-826 (1982) discloses that, by being provided
with "side tunnel regions" on both sides of a core region, not only high normalized
birefringence is realized, but also the cutoff frequency difference between two polarization
modes is enlarged, whereby an absolutely single-polarization optical fiber can be
realized.
[0005] USP 5,907,652 discloses the following air-clad optical fiber. Namely, it is a silica-based
optical fiber comprising, successively from the fiber center toward the outer periphery,
a core region, an inner cladding region, a first outer cladding region, and a second
outer cladding region, in which the refractive index of inner cladding is less than
that of the core region, and the effective refractive index of first outer cladding
region is less than 1.35. Also, the first outer cladding region is selected such that
optical characteristics of the optical fiber do not depend on the second outer cladding
region. It discloses that the air-clad optical fiber is suitable for cladding-pumped
optical fiber lasers and long-period fiber gratings.
Disclosure of the Invention
[0006] In the microstructured optical fiber disclosed in JP 10-95628A, however, microstructures
are distributed over the whole cladding, whereby the number of microstructures is
large. For example, the above-mentioned publication states that "Inventors' simulations
indicate that at least 4 layers of second capillary features should be provided."
In this case, the number of capillary features would be at least 90, thus becoming
large. If the number of microstructures is large as such, then the making becomes
difficult. According to the above-mentioned publication, the process of making the
microstructured optical fiber is as follows. Namely, silica capillary tubes and a
silica rod with no bore are prepared, a tube bundle is formed by arranging a number
of silica tubes around the silica rod, the tube bundle and an over cladding tube are
collapsed so as to yield a preform, and then an optical fiber is drawn from this preform.
However, it takes time and effort to form a tube bundle by arranging small-diameter
silica tubes into abundle without disorder. Also, since there is a strong possibility
of the arrangement being disordered, the making with a favorable reproducibility is
hard to achieve. The making becomes more difficult as the number of microstructures
increases.
[0007] On the other hand, a step of boring a preform of a conventional impurity-doped type
optical fiber by use of a boring device may be used instead of the above-mentioned
making process. Even in the case using this step, however, the conventional microstructured
optical fiber has a large number of microstructures, whereby the cost of manufacture
becomes high.
[0008] Also, the optical fiber disclosed in the above-mentioned publication has problems
as follows in particular when the microstructures are bores. First, the mechanical
strength of optical fiber is lowered due to the bores included therein, whereby strengths
against tension and lateral pressures may decrease. Second, there is a possibility
of absorption loss occurring due to OH group on surfaces of bores and water vapor
within the bores. Therefore, during operations of making or fiber connection, a treatment
for lowering the possibility of water vapor entering the bores is necessary, which
makes the operations difficult. Third, if glass melts upon fusion splicing and thereby
closes bores, then the effective refractive index difference between the core and
cladding is lost, so that the optical power leaking out into the cladding remarkably
increases, whereby propagation loss becomes greater at the fused part. The first and
second problems become more influential as the number of microstructures increases.
[0009] On the other hand, the refractive index difference realizable in the impurity-doped
type optical fiber disclosed in OFC'96 Technical Digest, ThA3 is small. As a result,
realizable value ranges are restricted in terms of the magnitude of absolute value
of negative dispersion, magnitude of absolute value of negative dispersion slope,
size of effective core area, and reduction of bending loss.
[0010] The optical fiber disclosed in Electronics Letters, vol. 18, pp. 824-826 (1982) yields
a large linear birefringence with "side tunnel regions" of air provided on both sides
of its core. However, it is desirable that birefringence be smaller for optical transmission
application, such as those in which the optical fiber is incorporated in a part of
an existing optical transmission line in particular. If the polarization state of
light incident on an optical fiber having a large birefringence does not match either
of the principal polarization states of fiber, then transmission quality deteriorates
due to polarization mode dispersion. Hence, a device for making the polarization state
of incident light constant is necessary, which raises the cost. Also, most of existing
optical transmission lines have no polarization selectivity, whereby the polarization
state of light emitted therefrom is inconstant. Thus, the polarization state of light
having an inconstant polarization state is hard to keep constant.
[0011] In the air-clad optical fiber disclosed in USP 5,907,652, a chromatic dispersion
having a large negative value and a chromatic dispersion slope having a large negative
value are hard to obtain. This is because of the fact that this optical fiber is mainly
aimed at lowering the effective refractive index of first outer cladding region, so
as to prevent the second outer cladding region from influencing optical characteristics.
[0012] In view of such circumstances, it is an object of the present invention to provide
an optical fiber which can realize a low chromatic dispersion (having a large negative
value), a low chromatic dispersion slope(having a large negative value), a large effective
core area, and a low bending loss. It is another object of the present invention to
provide an optical fiber facilitating its making, cutting down its cost, improving
its strengths against tension and lateral pressures, lowering the possibility of absorption
loss occurring due to OH group on surfaces of bores and water vapor within bores,
and reducing power loss at fusion splice.
[0013] For satisfying the above-mentioned objects, the present invention provides an optical
fiber comprising a core region constituted by a substantially homogeneous medium;
an inner cladding region surrounding the core region; and an outer cladding region,
constituted by a substantially homogeneous medium, surrounding the inner cladding
region; wherein the core region, inner cladding region, and outer cladding region
are regions extending along a fiber axis and influencing optical characteristics;
wherein an average refractive index n
0 of the core region, an average refractive index n
1 of the inner cladding region, and an average refractive index n
2 of the outer cladding region satisfy the relationship of
n1<n2<n0; and wherein the inner cladding region includes at least three regions each extending
along the fiber axis and comprising an auxiliary medium having a refractive index
different from that of a main medium constituting the inner cladding region.
[0014] Within a cross section perpendicular to the fiber axis, the core region has a substantially
circular form, whereas the inner and outer cladding regions have substantially annular
forms. The average refractive index of each of the core region, inner cladding region,
and outer cladding region can be given by the following n
avg:
where a is the inner radius of the region (0 in the case of core region), b is the
outer radius, r and θ are polar coordinates representing a position within a fiber
cross section, and n(r, θ) is a refractive index distribution within the cross section.
In general, the average refractive indices in core region, inner cladding region,
and outer cladding region vary depending on the respective definitions of regions.
The expressions "comprising a core region constituted by a substantially homogeneous
medium; an inner cladding region surrounding the core region; and an outer cladding
region, constituted by a substantially homogeneous medium, surrounding the inner cladding
region" and "an average refractive index n
0 of the core region, an average refractive index n
1 of the inner cladding region, and an average refractive index n
2 of the outer cladding region satisfy the relationship of
n1 <n2 <n0" mean that there is such a way of defining the core region, inner cladding region,
and outer cladding region as to satisfy the above-mentioned inequality. For improving
the fiber strength, the outer cladding region may be surrounded with a jacket region
made of a material such as glass or resin. Here, it is necessary for the outer cladding
region to have a sufficient radial thickness in order to prevent the jacket region
from influencing the optical characteristic. On the other hand, the outer cladding
region is a region influencing the optical characteristic, whereby the average refractive
index and thickness of the inner cladding region are selected such that the outer
cladding region influences the optical characteristic.
[0015] Each of the core region and outer cladding region is constituted by a substantially
homogeneous medium. It means that the main ingredient of the material constituting
eachof these regions is same in the respective region. Here, a configuration in which
the impurity concentration is varied within the region can be employed when appropriate.
For example, the core region may be silica glass containing Ge as an impurity while
employing a structure in which Ge concentration decreases from the center toward the
outer periphery.
[0016] The main medium is a medium which can actually constitute the optical fiber by itself.
Aplurality of main medium regions which are not connected together must not exist
in a single optical fiber. On the other hand, the auxiliary medium may be a medium
which cannot actually constitute the optical fiber by itself. A plurality of auxiliary
medium regions disconnected from each other may exist in a single optical fiber. A
typical example of the main medium is silica-based glass, whereas typical examples
of the auxiliary medium are gases or liquids.
[0017] Thus, in addition to a main medium constituting the inner cladding region, regions
constituted by an auxiliary medium having a refractive index different from that of
the main medium (hereinafter called auxiliary-medium regions) are provided in the
inner cladding region of the optical fiber in accordance with the present invention.
On the other hand, the outer cladding region is constituted by a substantially homogeneous
medium and includes no auxiliary-medium regions. This is based on the inventor's discovery
that, for yielding a favorable characteristic such as a dispersion having a large
negative value in an optical fiber in which the average refractive index of the inner
cladding region is less than that of the outer cladding region, it will be sufficient
if the average refractive index of the inner cladding region is lowered by providing
auxiliary-medium regions therein without necessitating providing auxiliary-medium
regions in the outer cladding region. On the other hand, by being provided with regions
each of which consists of an auxiliary medium having a refractive index less than
that of the main medium, the average refractive index of inner cladding region can
be made much lower than that in the case without the auxiliary-medium regions. As
a result, favorable characteristics such as a dispersion with a larger negative value,
a dispersion slope with a larger negative value, a larger effective core area, and
a smaller bending loss can be obtained as compared with the conventional impurity-doped
type optical fiber. Also, unlike the air-clad optical fiber, the optical fiber of
the present invention can realize a dispersion with a large negative value and a dispersion
slope with a large negative value. This is because of the fact that the outer cladding
region surrounding the inner cladding region including auxiliary-medium regions influences
optical characteristics, such as chromatic dispersion characteristics in particular.
Further, since the outer cladding region is constituted by a substantially homogeneous
medium and includes no auxiliary-medium regions, the number of auxiliary-medium regions
to be provided can be drastically reduced compared to that in the conventional microstructured
optical fiber. As a result, the optical fiber can easily be made with a favorable
reproducibility using any of the method in which silica tubes are arranged and the
method in which a preform is bored by use of a boring device, whereby the cost of
manufacture can be cut down.
[0018] In the case where the auxiliary-medium regions comprise bores in particular, strengths
against tension and lateral pressures improve as compared with the conventional microstructured
optical fiber because the number of auxiliary-medium regions decreases, and the making
and connecting become easier since the probability of absorption loss occurring due
to OH group on surfaces of bores and water vapor within the bores decreases. Further,
since the refractive index of core region is higher than that of outer cladding region,
the waveguiding function of the fiber will not be lost even if bores are closed in
the inner cladding, whereby attenuation due to fusion splice can be reduced.
Brief Description of the Drawings
[0019]
Fig. 1 is a sectional view of an optical fiber in accordance with the first embodiment
of the present invention;
Fig. 2 is a sectional view of a conventional impurity-doped type optical fiber for
comparison;
Fig. 3 is a chart showing relationships between the mode field diameter and the chromatic
dispersion at a wavelength of 1550 nm;
Fig. 4 is a chart showing relationships between the mode field diameter and the chromatic
dispersion slope at a wavelength of 1550 nm;
Fig. 5 is a chart showing relationships between the ratio of optical power propagating
through the outer cladding region to the total power and the chromatic dispersion
and chromatic dispersion slope at a wavelength of 1550 nm;
Fig. 6 is a chart showing a relationship between the ratio of optical power propagating
through the outer cladding region to the total power and the V value of core region;
Fig. 7 is a sectional view of an optical fiber in accordance with the second embodiment
of the present invention;
Fig. 8 is a sectional view of an optical fiber in accordance with the second embodiment
of the present invention;
Fig. 9 is a chart showing results of calculation of relationships between the light
wavelength λ, chromatic dispersion D, and effective core area Aeff in optical fibers in accordance with the second embodiment;
Fig. 10 is a schematic diagram of an optical communication system including an optical
fiber in accordance with the second embodiment as a negative dispersion optical fiber;
Fig. 11 is a sectional view of an optical fiber in accordance with the third embodiment
in the fiber axis direction;
Fig. 12A is a sectional view of the optical fiber taken along the line I-I in Fig.
11;
Fig. 12B is a sectional view of the optical fiber taken along the line II-II in Fig.
11;
Fig. 13 is a chart showing results of numerical simulation of chromatic dispersion
characteristics in segments a and b in the optical fiber in accordance with the third
embodiment;
Fig. 14 is a chart showing the average chromatic dispersion Davg in the case where a segment a having a length of 0.48 is combined with a segment
b having a length of 1 in the optical fiber in accordance with the third embodiment;
Fig. 15A is a sectional view of an optical fiber in accordance with the fourth embodiment
in its segment a;
Fig. 15B is a sectional view of the optical fiber in accordance with the fourth embodiment
in its segment b;
Fig. 16 is a chart showing results of numerical simulation of chromatic dispersion
characteristics in segments a and b in the optical fiber in accordance with the fourth
embodiment; and
Fig. 17 is a chart showing the average chromatic dispersion Davg in the case where a segment a having a length of 0.42 is combined with a segment
b having a length of 1 in the optical fiber in accordance with the fourth embodiment.
Best Modes for Carrying Out the Invention
[0020] In the following, embodiments of the present invention will be explained with reference
to the drawings.
(First Embodiment)
[0021] Fig. 1 is a sectional view of an optical fiber 10B in accordance with the first embodiment.
A core region 11 of this optical fiber 10B has a circular form with a radius α constituted
by silica glass (whose refractive index n
0 = 1.46567) doped with Ge having a concentration of 14.5 mol%, whereas a cladding
region 12 having an outside radius of γ made of pure silica glass (whose refractive
index n
2 = 1.44402) surrounds the core region 11. The cladding region 12 is constituted by
an inner cladding region 14 surrounding the core region 11 and having auxiliary-medium
regions 13 (whose refractive index n
3 = 1), and an outer cladding region 15 surrounding the inner cladding region 14 and
including no auxiliary-medium regions 13. The main medium of the inner cladding region
14 is pure silica glass, whereas the auxiliary medium forming the auxiliary-medium
regions 13 is air. Eight auxiliary-medium regions 13, each having a circular formwith
a radius r, are disposed on a circle having a radius β' at substantially equal intervals.
The outer side of the outer cladding region 15 is coated with a jacket layer constituted
by a material such as glass or polymer. While the jacket layer is aimed at improving
mechanical performances so as to restrain microbend from occurring and improve the
strength of fiber, for example, the outer cladding region 15 is so thick that the
influence of the jacket layer upon optical characteristics is negligible. The boundary
between the inner cladding region 14 and outer cladding region 15 is defined by a
circle having a radius of β=2β'-α (which indicates that β' is the mean of β and α,
i.e., the center of microstructures 13 is located at a position equally distanced
in a radial direction from the boundary between the inner cladding region 14 and core
region 11 and from the boundary between the inner cladding region 14 and outer cladding
region 15.
[0022] Structural parameters of the optical fiber 10B are as follows: β'/α = 1.94, r/α =
0.135, γ/α = 18.3.
[0023] Here, letting a be the inside radius of each region (0 in the case of core region)
and b be the outside radius thereof, the position within a fiber cross section is
represented by polar coordinates, whereby the refractive index distribution within
the cross section using the polar coordinates is given by n(r, θ). Then, the average
refractive index n
avg of the region is given by the following equation:
[0024] From this equation, the following expression can simply give an average refractive
index n
avg in a predetermined region constituted by a main medium region having a uniform refractive
index n
m, regions formed by an auxiliary medium having a refractive index n
s different from the refractive index n
m exists:
where A
m and A
s are cross areas of the main medium region and the microstructures, respectively.
[0025] From expression (2) and the above-mentioned individual parameters, the average refractive
index n
1 of inner cladding region 14 becomes 1.4366.
[0026] Fig. 2 is a sectional view of a conventional impurity-doped type optical fiber 10A
for comparison. In the optical fiber 10A, the material of core region 11 is silica
having a Ge concentration of 14.5 mol%, the material of inner cladding 14 is silica
having an F concentration of 1.113 wt%, and the material of outer cladding region
15 is pure silica.
[0027] Structural parameters of the optical fiber 10A are as follows: β/α = 2.88, γ/α =
18.3.
[0028] Figs. 3 to 6 are charts showing changes in optical characteristics in the optical
fibers 10A and 10B when mode field diameter is made variable by changing sizes while
keeping the ratios between each dimension constant. The abscissa in each of Figs.
3 and 4 indicates the mode field diameter MFD, the ordinate in Fig. 3 indicates the
chromatic dispersion D
1550 at a wavelength of 1550 nm, and the ordinate in Fig. 4 indicates the chromatic dispersion
slope S
1550 at a wavelength of 1550 nm. Fig. 5 shows relationships between the ratio P
oc of optical power propagating through the outer cladding region in the optical fiber
10B and optical characteristics. In Fig. 5, the abscissa indicates the ratio P
oc of optical power propagating through the outer cladding region to the total power,
whereas the left and right ordinates indicate the chromatic dispersion D
1550 and chromatic dispersion slope S
1550 at a wavelength of 1550 nm, respectively. Fig. 6 shows the relationship between the
ratio P
oc of optical power propagating through the outer cladding region to the total power
in the optical fiber 10B and fiber size, in which the abscissa and ordinate indicate
the V value of core and the ratio P
oc of optical power, respectively. Here, the V value of core is a value proportional
to size, and is defined by:
where n
0 and n
2 are respective refractive indices of the core and outer cladding, and k is the wave
number in vacuum.
[0029] Figs. 3 and 4 indicate that the optical fiber 10B has negative dispersion and dispersion
slope with greater absolute values than those of the optical fiber 10A, respectively.
For example, when MFD = 7 µm, while D
1550 = -90 ps/nm/km and S
1550 = -0.25 ps/nm
2/km in the optical fiber 10A, D
1550 = -107 ps/nm/km and S
1550 = -0.84 ps/nm
2/km in the optical fiber 10B. Since the negative dispersion and chromatic dispersion
slope have greater absolute values, the length required for compensating for positive
dispersion and chromatic dispersion slope can be made shorter, whereby the optical
fiber 10B can be considered more suitable for compensating for positive dispersion
and chromatic dispersion slope than the optical fiber 10A.
[0030] Fig. 5 indicates that a negative dispersion and a negative dispersion slope can be
obtained when the ratio P
oc of optical power propagating through the outer cladding region to the total power
is 0.008 or greater. It also indicates that negative dispersion and negative dispersion
slope with particularly greater absolute values can be obtained when the ratio P
oc of optical power propagating through the outer cladding region to the total power
is 0.1 or greater. As shown in Fig. 6,
Vcore ≤ 1.63 in order to realize that
Poc ≥ 0.008, and
Vcore ≤ 1.34 in order to realize that P
oc≥0.1.
[0031] As explained in the foregoing, unlike the conventional air-clad optical fiber, the
optical fiber 10B in accordance with the first embodiment can realize a low chromatic
dispersion (having a large negative value) and a low chromatic dispersion slope (having
a large negative value) . Also, its chromatic dispersion and chromatic dispersion
slope have greater negative values than those in the impurity-doped type optical fiber
10A. Therefore, it is suitable for compensating for the positive chromatic dispersion
and positive chromatic dispersion slope of optical transmission line. Also, its birefringence
is low. Further, since the glass refractive index of core is higher than the glass
refractive index of cladding unlike the conventional microstructured optical fiber,
the splicing loss caused by collapsing of bores at the time of fusion is low. Also,
since the number of bores is small, i.e., 8, the making is easy, and strength is high.
In particular, negative chromatic dispersion and negative chromatic dispersion slope
having greater absolute values are obtained when the ratio of power propagating through
the outer cladding to the total power is 0.1 or greater.
(Second Embodiment)
[0032] Figs. 7 and 8 are sectional views of optical fibers 10E, 10F, and 10G in accordance
with the second embodiment of the present invention. A core region 30 and an inner
cladding region 31 are formed by silica glass doped with Ge (having a refractive index
n
0), whereas the inner cladding region 31 includes a plurality of auxiliary-medium regions
32 (having a refractive index n
3). In the inner cladding region 31, the main medium is Ge-doped silica glass (n
0 = 1.46567), whereas the auxiliary medium forming the microstructures 32 is air (n
3 = 1). The microstructures 32, each having a circular form with a radius r, are disposed
on a circle having a radius β' at substantially equal intervals. The outer periphery
of inner cladding region is a circle having a radius β. The boundary between the core
region 30 and inner cladding region 31 is defined by a circle having a radius of α=2β-β'.
The outer cladding region 33 is formed from pure silica glass. The impurity-doped
type optical fiber 10A shown in Fig. 2 is used for comparison.
[0033] Structural parameters of the optical fibers 10E and 10F shown in Fig. 7 are as follows.
Namely, for the optical fiber 10E, α = 1.02 µm, β' = 1.97 µm, and r = 0.253 µm. Here,
the average refractive index n
1 of inner cladding region 31 is 1.43883. For the optical fiber 10F, α = 1.25 µm, β'
= 1.87 µm, and r = 0.215 µm. Here, the average refractive index n
1 of inner cladding region 31 is 1.43395.
[0034] For the optical fiber 10G shown in Fig. 8, α = 1.50 µm, β' = 1.84 µm, and r = 0.155
µm. Here, the average refractive index n
1 of inner cladding region 31 is 1.4211.
[0035] Fig. 9 shows results of calculation of relationships between light wavelength λ,
chromatic dispersion D, and effective core area A
eff in the optical fibers 10E to 10G having the foregoing structures in accordance with
the second embodiment. In this chart, the abscissa, left ordinate, and right ordinate
indicate the light wavelength λ, chromatic dispersion D, and effective core area A
eff, respectively. While the effective core area A
eff = 30 µm
2 in each of the optical fibers 10A, 10E, 10F, and 10G, the chromatic dispersion D
successively increases so as to become -155 ps/nm/km in the optical fiber 10A, -164
ps/nm/km in the optical fiber 10E, -208 ps/nm/km in the optical fiber 10F, and -254
ps/nm/km in the optical fiber 10G. The rate at which the effective core area A
eff increases relative to the increase in wavelength is lower in the optical fibers 10E
and 10F than the rate at which the effective core area A
eff increases relative to the increase in wavelength in the optical fiber 10A. The fact
that the rate at which the effective core area A
eff increases relative to the increase in wavelength is lower means that light is more
tightly confined in the core, whereby the bending loss is low. Also, since the bending
loss increases if the effective core area A
eff is made larger, each of the optical fibers 10E and 10F can realize an effective core
area A
eff greater than that in the optical fiber 10A with the same bending loss. Also, since
the arrangement of auxiliary-medium regions in each of the optical fibers 10E, 10F,
and 10G substantially has a quarter rotational symmetry, two polarization modes degenerate,
whereby mode birefringence is low.
[0036] Therefore, the optical fibers 10E to 10G in accordance with the second embodiment
can realize a smaller chromatic dispersion (having a larger negative value) , a lower
bending loss, and a greater effective core area as compared with the impurity-doped
type optical fiber 10A shown in Fig. 2. Since the chromatic dispersion has a larger
negative value, the length required for compensating for a positive dispersion is
shorter, and the effective core area is greater. Therefore, if an optical fiber in
accordance with the second embodiment is used as a negative dispersion optical fiber
in an optical communication system including an optical transmitter 50, an optical
receiver 51, a positive dispersion optical fiber 52, and a negative dispersion optical
fiber 53 as shown in Fig. 10, then the nonlinear optical effects in the negative dispersion
optical fiber which deteriorates the transmission line quality can be suppressed,
whereby a large-capacity optical communication system can be realized.
[0037] In addition to a main medium constituting an inner cladding region, regions made
of an auxiliary medium having a refractive index different from that of the main medium
are provided in the inner cladding region of the optical fiber in accordance with
the present invention. On the other hand, the outer cladding region is constituted
by a substantially homogeneous medium and includes no auxiliary-medium regions. Since
regions made of the auxiliary medium having a refractive index lower than that of
the main medium are provided, the average refractive index of inner cladding region
can be made much lower than that in the case without the auxiliary-medium regions.
As a result, favorable characteristics such as a dispersion with a larger negative
value, a dispersion slope with a larger negative value, a larger effective core area,
and a smaller bending loss can be obtained as compared with the conventional impurity-doped
type optical fiber. Also, unlike the air-clad type optical fiber, the optical fiber
of the present invention can realize a dispersion with a large negative value and
a dispersion slope with a large negative value. This is because of the fact that the
outer cladding region surrounding the inner cladding region including auxiliary-medium
regions influences optical characteristics, chromatic dispersion characteristics in
particular. Further, since the outer cladding region is constituted by a substantially
homogeneous medium and includes no auxiliary-medium regions, the number of auxiliary-medium
regions to be provided can be drastically reduced compared to that in the conventional
microstructured optical fiber. As a result, the optical fiber can easily be made with
a favorable reproducibility using any of the method in which silica tubes are arranged
and the method in which a preform is bored by use of a boring device, whereby the
cost of manufacture can be cut down.
[0038] In the case where the auxiliary-medium regions comprise bores in particular, strengths
against tension and lateral pressures improve as compared with the conventional microstructured
optical fiber when the number of auxiliary-medium regions decreases, and the making
and connecting become easier since the probability of absorption loss occurring due
to OH group on surfaces of bores and water vapor within the bores decreases. Further,
since the refractive index of core region is higher than that of outer cladding region,
optical waveguide characteristics will not be lost even if bores are collapsed in
the inner cladding, whereby fusion loss can be reduced.
[0039] The auxiliary-medium regions maybe arranged such that a quarter rotational symmetry
about the fiber axis substantially holds. As a consequence, two polarization modes
can substantially degenerate, whereby birefringence can be made lower. Also, they
may be arranged at substantially equal intervals on at least one of concentric circles
centered at the fiber axis. As a consequence, two polarization modes can substantially
degenerate, whereby birefringence can be made lower. Arranging the auxiliary-medium
regions along a circle can also yield an effect approximately equivalent to uniformly
changing the refractive index of an annular region including this circle. Therefore,
designing can be made according to radial refractive index profiles as in the conventional
impurity-doped optical fiber. As a consequence, systematic designing becomes easier.
The microstructures may also be disposed at substantially equal intervals on a circle
centered at the fiber axis. As a consequence, two polarization modes can substantially
degenerate, whereby birefringence can be made lower. Also, systematic designing becomes
easier. Further, since the number of microstructures is minimized, easiness in the
making, high strength, and high reliability can be realized.
[0040] In the optical fiber in accordance with the present invention, the ratio of optical
power propagating through the outer cladding region to the total power at a wavelength
of 1550 nm can be made 0.008 or greater (more preferably 0.1 or greater). As a consequence,
the outer cladding region can become not only a region for improving mechanical strength
and so forth, but also a region actually influencing optical characteristics (chromatic
dispersion characteristics in particular) of the optical fiber. In particular, when
the ratio of optical power propagating through the outer cladding region to the total
power is 0.008 or greater, a lower chromatic dispersion slope (having a larger negative
value) canbe realized. Also, when the ratio of optical power propagating through the
outer cladding region to the total power is 0.1 or greater, a lower chromatic dispersion
slope (having a larger negative value) can be realized.
[0041] Further employable is a configuration in which each of the medium of core region,
the main medium of inner cladding region, and the medium of outer cladding region
is silica-based glass which may be doped with impurities, whereas the auxiliary medium
forming the auxiliary-medium regions in the inner cladding region is gas or vacuum.
As a result, transmission loss can be kept low, whereas the average refractive index
of inner cladding can be lowered greatly, so as to realize favorable characteristics
such as a dispersion having a larger negative value than that in the conventional
impurity-doped type optical fiber.
(Third Embodiment)
[0042] Fig. 11 is a sectional view of an optical fiber 10H in accordance with the third
embodiment of the present invention in the fiber axis direction. Fig. 12A is a sectional
view of the optical fiber taken along the line I-I in Fig. 11, whereas 12B is a sectional
view of the optical fiber taken along the line II-II in Fig. 11. In the optical fiber
10H in accordance with the third embodiment, segments a and b are alternately disposed
in the fiber axis direction. While the segment a includes bores 43 in its inner cladding
region 44, the segment b includes no bores 43. A transition segment c exists between
the segments a and b, whereas the cross sectional area of bore changes along the fiber
axis in the transition segment c. Typically, each length of the segments a and b is
at least 100 m. On the other hand, a length of the transition segment c can be 1 m
or shorter. Here, the influence of optical characteristics of the transition segment
c upon optical characteristics of the whole optical fiber is negligible. The diameter
of core region 41 is 2α, which is the same value in the segments a and b. In the segment
a, as shown in Fig. 12A, eight bores 43 (each having a radius of r) are arranged at
equally spaced intervals on a circle having a radius β' centered at the fiber axis.
The refractive index distribution in the segment a corresponds to that having a depressed
portion which is an annular region including the bores 43, whereas the refractive
index distribution in the segment b corresponds to that without the depressed portion.
As in the first and second embodiments, the outside radius of inner cladding region
44 is set as β=2β'-α, and the outside radius of outer cladding region 45 is γ.
[0043] Structural parameters of the optical fiber 10H are as follows: α = 1.70 µm, β' =
2.74 µm, r = 0.25 µm. The core region 41 is silica having a Ge concentration of 12
mol%, the main medium of inner cladding region 44 is silica having a Ge concentration
of 5.0 mol%, and the outer cladding region 45 is pure silica. While the segment a
includes the bores 43, the segment b includes no bores 43, whereby the average refractive
index n
1 of inner cladding region 44 is 1.435 in the segment a, and 1.452 in the segment b,
thus varying along the fiber axis.
[0044] Fig. 13 is a chart showing results of numerical simulation of chromatic dispersion
D in the segments a and b in the optical fiber 10H. Here, as shown in Fig. 13, the
wavelength ranges from 1510 nm to 1600 nm. The segment a has a negative chromatic
dispersion and a negative chromatic dispersion slope, whereas the segment b has a
positive chromatic dispersion and a positive chromatic dispersion slope. In particular,
the chromatic dispersion D and chromatic dispersion slope S at a wavelength of 1550
nm are:
and
whereas
and
[0045] At 1550 nm, the ratio of optical power P
oc propagating through the outer cladding region to the total power is 0.048.
[0046] Fig. 14 is a chart showing the average chromatic dispersion D
avg in the case where a fiber is constituted by segments a having total length of 0.48,
and segments b having total length of 1. Here, when a fiber is constituted by fiber
segments i (i = 1, 2, ..., n) having chromatic dispersions D
i and lengths L
i, the average chromatic dispersion D
avg is defined by the following expression. Also, letting L be the length of the fiber,
the total chromatic dispersion is defined by D
avgL.
[0047] Similarly, when a fiber is constituted by fiber segments i (i = 1, 2, ..., n) having
chromatic dispersion slopes S
i and lengths L
i, the average chromatic dispersion slope S
avg is defined by the following expression. Also, the total chromatic dispersion slope
is defined by S
avgL.
[0048] The chromatic dispersion in a fiber segment where the chromatic dispersion can be
considered constant is referred to as local chromatic dispersion. It is thus defined
so as to be distinguishable from the total chromatic dispersion in the whole transmission
line constituted by such fiber segments.
[0049] As shown in Fig. 14, the average chromatic dispersion D
avg and average chromatic dispersion slope S
avg become substantially zero at a wavelength of 1550 nm. As a consequence, in an optical
fiber transmission line having segments a and b at the above-mentioned ratio, the
absolute value of average chromatic dispersion becomes 0.4 ps/nm/km or less in a wide
wavelength band of 1510 nm to 1600 nm. On the other hand, as shown in Fig. 13, the
absolute value of local chromatic dispersion is large, i.e., 4 ps/nm/km or greater.
Though the absolute value of local chromatic dispersion becomes smaller in a part
of fiber segment included in the transition segment, the length of such a fiber segment
can be made shorter (e.g., 1 m or less), whereby the magnitude of influence of nonlinear
optical phenomena in the transition segment is negligible. Therefore, the distortion
of optical pulse due to cumulative dispersion and the deterioration in transmission
quality due to nonlinear optical phenomena caused by optical signals having different
wavelengths can be suppressed at the same time.
[0050] Thus, in the optical fiber in accordance with the third embodiment, it is possible
to change the refractive index distribution drastically within the fiber cross section
along the fiber axis, thereby greatly altering chromatic dispersion relative to the
wavelength along the fiber axis. As a consequence, chromatic dispersion characteristics
which are hard to realize or impossible in an optical fiber constituted by a single
kind of fiber segments can be realized. In particular, characteristics with a large
absolute value of local chromatic dispersion and a small absolute value of total chromatic
dispersion can be realized.
[0051] As compared with conventional dispersion management fibers, in the optical fiber
in accordance with this embodiment, it is possible to change the refractive index
distribution within the fiber cross section more drastically along the fiber axis,
thereby altering chromatic dispersion relative to the wavelength more greatly along
the fiber axis. Therefore, it is possible to realize an optical fiber having a fiber
segment a whose chromatic dispersion is lower than -10 ps/nm/km in a wavelength band
of 1510 nm to 1600 nm and a fiber segment b whose chromatic dispersion is higher than
+5 ps/nm/km in this wavelength band, in which the absolute value of average chromatic
dispersion in this wavelength band is less than 0.4 ps/nm/km, the chromatic dispersion
slope of fiber segment a is negative in this wavelength band, and the chromatic dispersion
slope of fiber segment b is positive in this wavelength band. As a result, as compared
with the prior art, the wavelength range where the absolute value of total chromatic
dispersion becomes lower than a predetermined value can be widened, so as to enhance
the transmission capacity.
[0052] Further, in the optical fiber in accordance with this embodiment, a plurality of
segments b including no bores are arranged at intervals along the fiber axis. As a
result, the optical fiber can be cleaved at the segments b, so as to be fusion-spliced
to other optical fibers. Here, unlike the conventional microstructured optical fiber,
the problems of the deform or disappearance of auxiliary-medium regions due to fusion
and block for observing the core due to auxiliary-medium regions do not occur, whereby
the fusion splicing becomes easier than that in the conventional microstructured optical
fiber. Also, there are no bores open to the outside air at end faces, whereby no contaminants
enter the bores. Therefore,low-lossmechanicalconnection can be realized by use of
a refractive index matching liquid. Further, even if a side face is damaged in a part
of fiber segments a and thereby a contaminant such as water enters bores, the contaminant
will not spread over the whole fiber, whereby the tolerance to damages is higher than
that in the conventional microstructured optical fiber.
(Fourth Embodiment)
[0053] Figs. 15A and 15B are sectional views of an optical fiber 10I in accordance with
the fourth embodiment of the present invention in segments a and b thereof, respectively.
In the optical fiber 10I in accordance with the fourth embodiment, as in the optical
fiber 10H in accordance with the third embodiment, segments a and b are alternately
disposed along the fiber axis. While the segment a includes bores 53 in its inner
cladding region 54, the segment b includes no bores 53. A transition segment c exists
between the segments a and b, whereas the cross sectional area of bore changes along
the fiber axis in the transition segment c. Typically, each length of the segments
a and b is at least 100 m. On the other hand, a length of the transition segment c
can be 1 m or shorter. Here, the influence of optical characteristics of the transition
segment c upon optical characteristics of the whole optical fiber is negligible. The
diameter of core region 51 is 2α, which is the same value in the segments a and b.
In the segment a, as shown in Fig. 15A, eight bores 53 (each having a radius of r)
are arranged at equally spaced intervals on a circle having a radius β' centered at
the fiber axis. The refractive index distribution in the segment a corresponds to
that having a depressed portion which is an annular region including the bores 53,
whereas the refractive index distribution in the segment b corresponds to that without
the depressed portion. As in the first to third embodiments, the outside radius of
inner cladding region 54 is set as β=2β'-α.
[0054] Structural parameters of the optical fiber 10I are as follows: α = 1.74 µm, β' =
2.81 µm, r = 0.39 µm. The core region 51 is silica having a Ge concentration of 14
mol% (whose refractive index n
0 = 1.465), the main medium of inner cladding region 54 and the outer cladding region
55 are pure silica (whose refractive index n
2 = n
3 = 1.444). While the segment a includes the bores 53, the segment b includes no bores
53, whereby the average refractive index n
1 of inner cladding region 54 varies along the fiber axis.
[0055] Fig. 16 is a chart showing results of numerical simulation of chromatic dispersion
in the segments a and b in the optical fiber 10I. Here, as shown in Fig. 16, the wavelength
ranges from 1510 nm to 1600 nm. The segment a has a positive chromatic dispersion
and a negative chromatic dispersion slope, whereas the segment b has a negative chromatic
dispersion and a positive chromatic dispersion slope. In particular, the chromatic
dispersion D and chromatic dispersion slope S at a wavelength of 1550 nm are:
and
whereas
and
[0056] At 1550 nm, the ratio of optical power P
oc propagating through the outer cladding region to the total power is 0.0081.
[0057] Fig. 17 is a chart showing the average chromatic dispersion D
avg in the case where a fiber is constituted by segments a having total length of 0.42
and segments b having total length of 1. The average chromatic dispersion D
avg and average chromatic dispersion slope S
avg become substantially zero at a wavelength of 1550 nm. As a consequence, in an optical
fiber transmission line having segments a and b at the above-mentioned ratio, the
absolute value of average chromatic dispersion becomes 1 ps/nm/km or less in a wide
wavelength band of 1510 nm to 1600 nm. On the other hand, as shown in Fig. 16, the
absolute value of local chromatic dispersion is large, i.e., 10 ps/nm/km or greater.
Therefore, the distortion of optical pulse due to total dispersion and the deterioration
in transmission quality due to nonlinear optical phenomena caused by optical signals
having different wavelengths can be suppressed at the same time.
[0058] Thus, the optical fiber in accordance with the fourth embodiment can also widely
change the refractive index distribution within the fiber cross section along the
fiber axis, thereby greatly altering the relationship of chromatic dispersion to wavelength
along the fiber axis. As a consequence, chromatic dispersion characteristics which
are hard to realize or impossible in an optical fiber constituted by a single kind
of fiber segments can be realized. In particular, characteristics with a large absolute
value of local chromatic dispersion and a small absolute value of total chromatic
dispersion can be realized.
[0059] By designing the wavelength characteristics of chromatic dispersion along fiber axis
appropriately, it is possible to realize an optical fiber having a fiber segment a
whose chromatic dispersion in a wavelength band of 1510 nm to 1600 nm is higher than
20 ps/nm/km and a fiber segment b whose chromatic dispersion in this wavelength band
is lower than -10 ps/nm/km, in which the absolute value of average chromatic dispersion
in this wavelength band is smaller than 1 ps/nm/km. As compared with conventional
dispersion management fibers, the optical fiber in accordance with this embodiment
can change the refractive index distribution within the fiber cross section more drastically
along the fiber axis, so as to alter chromatic dispersion relative to wavelength more
greatly along the fiber axis, thereby yielding a larger absolute value of local chromatic
dispersion in each segment. As a result, the distortion of optical pulse due to total
chromatic dispersion can be suppressed, and the deterioration in transmission quality
due to nonlinear optical phenomena caused by optical signals having different wavelengths
can be made lower than that in the prior art.
[0060] Further, as compared with conventional dispersion management fibers, the optical
fiber in accordance with this embodiment can change the refractive index distribution
within the fiber cross section more drastically along the fiber axis, thereby altering
wavelength characteristics of chromatic dispersion more greatly along the fiber axis.
Therefore, it is possible to realize an optical fiber having a fiber segment a whose
chromatic dispersion is higher than 20 ps/nm/km in a wavelength band of 1510 nm to
1600 nm and a fiber segment b whose chromatic dispersion is lower than -10 ps/nm/km
in this wavelength band, in which the absolute value of average chromatic dispersion
in this wavelength band is less than 1 ps/nm/km, the chromatic dispersion slope of
fiber segment a is negative in this wavelength band, and the chromatic dispersion
slope of fiber segment b is positive in this wavelength band. As a result, as compared
with the prior art, the wavelength range in which the absolute value of total chromatic
dispersion becomes lower than a predetermined value can be widened, so as to enhance
the transmission capacity.
[0061] As explained in the foregoing, the optical fiber in accordance with the present invention
comprises a core region constituted by a substantially homogeneous medium; an inner
cladding region surrounding the core region; and an outer cladding region, constituted
by a substantially homogeneous medium, surrounding the inner cladding region; the
core region, inner cladding region, and outer cladding region extending along the
fiber axis; the average refractive index n
0 of core region, the average refractive index n
1 of inner cladding region, and the average refractive index n
2 of outer cladding region satisfying the relationship of
n1 < n2 < n0 ; wherein at least three regions each extending along the fiber axis and comprising
an auxiliary medium having a refractive index different from that of a main medium
constituting the inner cladding region are included in the inner cladding region.
[0062] Such a configuration can make the average refractive index of inner cladding region
much lower than that in the case without the auxiliary-medium regions, whereby a larger
negative dispersion, a larger negative dispersion slope, a larger effective core area,
and a smaller bending loss can be obtained as compared with the conventional impurity-doped
type optical fiber. Also, the outer cladding region influences optical characteristics
unlike the air-clad type optical fiber, whereby the optical fiber of the present invention
can realize a dispersion with a large negative value and a dispersion slope with a
large negative value as compared with the conventional air-clad type optical fiber.
Further, since the number of auxiliary-medium regions to be introduced can greatly
be reduced, the optical fiber can be made easily with a favorable reproducibility,
and the cost of manufacture can be cut down. Also, as compared with the conventional
microstructured optical fiber, strengths against tension and lateral pressures improve,
and the probability of absorption loss occurring due to OH group on surfaces of bores
and water vapor within the bores decreases, whereby the making and connecting become
easier. Further, since the refractive index of core region is higher than that of
outer cladding region, the waveguiding function of the fiber will not be lost even
if bores are collapsed in the inner cladding, whereby attenuation due to the fusion
splice can be reduced.
Industrial Applicability
[0063] The optical fiber in accordance with the present invention can suitably be used as
an optical transmission line or dispersion-compensating fiber.
1. An optical fiber comprising a core region constituted by a substantially homogeneous
medium; an inner cladding region surrounding said core region; and an outer cladding
region, constituted by a substantially homogeneous medium, surrounding said inner
cladding region; wherein said core region, inner cladding region, and outer cladding
region are regions extending along a fiber axis and influencing optical characteristics;
wherein an average refractive index n
0 of said core region, an average refractive index n
1 of said inner cladding region, and an average refractive index n
2 of said outer cladding region satisfy the relationship of
and
wherein said inner cladding region includes at least three regions each extending
along said fiber axis and comprising an auxiliary medium having a refractive index
different from that of a main medium constituting said inner cladding region.
2. An optical fiber according to claim 1, wherein the ratio of optical power propagating
through said outer cladding region to the total power at a predetermined wavelength
is at least 0.008.
3. An optical fiber according to claim 2, wherein the ratio of optical power propagating
through said outer cladding region to the total power at said predetermined wavelength
is at least 0.1.
4. An optical fiber according to claim 2, wherein the number of regions formed of auxiliary
medium included in said inner cladding region is 50 or less.
5. An optical fiber according to claim 2, wherein said regions formed of auxiliary medium
in said inner cladding region are arranged such that a quarter rotational symmetry
about said fiber axis substantially holds.
6. An optical fiber according to claim 5, wherein said regions formed of auxiliary medium
in said inner cladding region are. arranged at substantially equal intervals on at
least one of concentric circles centered at said fiber axis.
7. An optical fiber according to claim 6, wherein said regions formed of auxiliary medium
in said inner cladding region are arranged at substantially equal intervals on a circle
centered at said fiber axis.
8. An optical fiber according to claim 2, wherein the chromatic dispersion of the fundamental
mode of said optical fiber is lower than -100 ps/nm/km.
9. An optical fiber according to claim 2, wherein, at a predetermined wavelength, said
optical fiber has a positive chromatic dispersion and a negative chromatic dispersion
slope.
10. An optical fiber according to claim 2, wherein saidmedium of core region, saidmain
medium of inner cladding region, and said medium of outer cladding region are pure
or doped silica glass; and wherein said auxiliary medium of inner cladding region
is gas or vacuum.
11. An optical fiber communication system comprising an optical transmitter, an optical
fiber transmission line, and an optical receiver; wherein said optical fiber transmission
line includes the optical fiber according to claim 2, and an optical fiber having
a chromatic dispersion with a polarity different from that of said optical fiber's
chromatic dispersion.
12. An optical fiber according to claim 2, wherein at least one of cross sectional area
and refractive index of said regions formed of auxiliary medium in said inner cladding
region changes along the fiber axis.
13. An optical fiber according to claim 12, comprising a first fiber segment having, at
a predetermined wavelength, a chromatic dispersion higher than a predetermined positive
value; and a second fiber segment having, at said wavelength, a chromatic dispersion
lower than a predetermined negative value.
14. An optical fiber according to claim 13, wherein said first fiber segment has a chromatic
dispersion higher than +1 ps/nm/km at said predetermined wavelength, said second fiber
segment has a chromatic dispersion lower than -1 ps/nm/km at said predetermined wavelength,
and the total length of fiber segments whose absolute value of chromatic dispersion
at said wavelength is less than 1 ps/nm/km is not greater than 1/10 of the total length
of optical fiber.
15. An optical fiber according to claim 13, wherein the chromatic dispersion slope of
said first fiber segment at said wavelength and the chromatic dispersion slope of
said second fiber segment at said wavelength have respective polarities different
from each other.
16. An optical fiber according to claim 2, wherein a plurality of fiber segments without
said auxiliary medium are disposed at intervals along the fiber axis.